This invention relates to a method for making a liquid foam of graphene. More particularly, this invention describes such a method that modifies graphene oxide in water to generate a liquid foam wherein the modified form of graphene oxide is the foaming agent. The invention is also the liquid foam itself. Other objects of the invention include materials made with the liquid foam of graphene such as composites, porous networks and solid cellular foams, [D. Alcazar. Liquid Foams of Graphene. PhD Thesis Massachusetts Institute of Technology (2012).] [Liquid Foams of Graphene, D. Alcazar, L Gibson and E. L. Thomas, submitted to Science, June, 2012].
Liquid foams are dispersions of bubbles in a liquid. Most liquid foams are based on surfactant agents. [A. Saint-Jalmes, D. J. Durian, D. A. Weitz. Foams. In: Kirk-Othmer Encyclopedia of Chemical Technology (2004) and D. Weaire, S. Hutzler. The Physics of Foams. Oxford University Press (1999).] To make aqueous foams, typical surfactants are amphiphilic molecules made of hydrophilic and hydrophobic parts combined into linear or branched chain molecular structures. [T. Tadros. Surfactants. In: Kirk-Othmer Encyclopedia of Chemical Technology (2006) and D. Myers. Surfactant Science and Technology. Wiley-Interscience (2006).] There is no prior art of as liquid foam made with a foaming agent having a sheet molecular structure.
Graphite is a layered material made of graphene molecules. Graphene is a one atom thick molecular sheet made, primarily, of sp2-hybridized carbon atoms where each carbon is bonded to three other carbons as shown in FIGS. 1(A) and 1(C). Graphene molecules are non-stoichiometric; they present a distribution of compositions, hence molecular weights, sizes and shapes. Therefore, the compositions of graphene and graphene derivatives are characterized in terms of average parameters such as the relative amounts of oxygen and carbon atoms, or the relative amounts of functional groups.
As used in this patent application, graphene oxide sheets have a heterogeneous chemical composition of fractional ratios of graphene and functionalized graphene regions as shown in FIG. 1(D). IPDA-treated graphene oxide is defined as treatment of graphene oxide with isophorone diamine (IPDA). The resulting material has increased graphene content as can be seen in FIG. 1(E). Other reducing agents (such as other amines) may be used.
An object of this invention is a liquid foam made with a foaming agent having a sheet molecular structure. In particular, the invention is such a liquid foam where air bubbles are encapsulated inside graphene shells and the shells are made of a modified form of graphene oxide. The modified form of graphene oxide is the foaming agents in such liquid foams of graphene.
Graphene has a unique combination of electrical, mechanical and barrier properties. [M. S. Fuhrer, C. Ning Lau, A. H. MacDonald. MRS Bulletin 35, 289 (2010).] This makes graphene a potentially competitive additive in the composite market, for example, in market segments where additives such as carbon blacks, carbon fibers and carbon nanotubes are currently in use. Therefore, the use of graphene as an additive for composites is desirable. In many applications it is desirable for the graphene to form a continuous phase or percolating network throughout a composite matrix, resulting in advantageous electrical conductivity of the composite and mechanical enhancement of the composite.
One way of using graphene as an additive in polymeric composites, for example, involves exfoliating graphene from its parent graphite and dispersing the exfoliated graphene in a polymeric matrix. Oxidation has been proposed as a scalable route to exfoliating graphite. The resulting graphite oxide is made of graphene oxide sheets, a form of graphene where oxygenated functional groups are covalently attached to the carbon atoms in the sheets as shown in FIG. 1(D). Graphite oxide readily exfoliates in water into stable dispersions of graphene oxide. [G. R. Hennig. Progress in Inorganic Chemistry 1, 125 (1959)] Graphene oxide, however, has poor electrical conductivity and it is likely to be more mechanically flexible than graphene. In order to have exfoliated graphene that has been dispersed in a polymeric matrix have sufficient electrical conductivity to avoid static charge buildup and to enhance mechanical stiffness and strength, for example, it is necessary to reduce graphene oxide to graphene.
Exfoliated graphene oxide dispersions have excellent stability in water. Therefore, reduction in an aqueous solution is preferable in order to maintain a high degree of exfoliation. However, the prior art for reducing graphene oxide in water has several shortcomings. Prior art techniques have made use of hazardous chemicals such as hydrazine that are not suitable components for composite formulations. Further, the aggregation of graphene oxide after it has been reduced often requires the use of stabilizers such as surfactants to minimize such aggregation. Filtration steps may also be required. Other drawbacks include the use of organic solvents, the use of sonication steps and the use of high temperatures.
An object of the present invention is a better process to reduce graphene oxide in water and to enable transfer of the resulting reduced graphene oxide out of the water phase to a polymeric resin phase. The present invention improves the current state of the art for reducing graphene oxide for subsequent manufacturing composites.
Porous materials are used in a multitude of applications, for example, as light weight separation media in filters and as support media in catalysis. An object of the present invention is an improved method to produce porous materials made of a modified form of graphene oxide.
Another object of the present invention is a method to prepare a solid cellular foam made by drying the liquid foam of graphene. The invention is also the solid foam itself. Such foam presents a closed-cell structure made by packing dried graphene shells. Such material is of interest as lightweight structure. Moreover, this solid graphene foam can act as a functional material given the ability of the graphene shells to encapsulate a fluid/gas and further release the liquid/gas cargo by action of a trigger such as by heating or by mechanical compression.
Yet another object of the invention is a method for making open cell foams that can be infiltrated with various materials such as sol-gels that are converted to ceramics, and monomers that can be polymerized to a polymer or crosslinked polymer network. It is contemplated that a second nonmiscible liquid be added to the water phase and to agitate the material to create a liquid/liquid closed or open cell foam to entrap either liquid.